The convergence of wireless and internet technologies has significantly changed the ways of using the internet and spurred the demand for a wide range of telematics applications.
The coexistence of various cellular networks such as CDMA, CDPD, GSM, and wireless LAN IEEE 802.11a/b/g offer a powerful mobile computing platform that supports mobility, completely freeing portable devices, such as laptop computers, personal digital assistants (PDA), cellular phones, etc. from being tethered with a wired connection. This convergence of technologies has created mobility and flexibility but has also introduced new problems that need scalable and feasible solutions.
The complexity of in-vehicle wireless internet stems from dynamically changing network environments and network heterogeneity. In an in-vehicle environment, a mobile host (MH) roams between base stations and switches coverage responsibility from one base station to another. The switch between base stations and networks is called a handoff and causes the MH to be temporarily disconnected. This is considered the primary source of dropped connections and performance deterioration. Handoff falls into two distinct categories; horizontal handoff and vertical handoff. A horizontal handoff refers to a switch between networks in a homogeneous network environment. A vertical handoff refers to a switch between networks in a heterogeneous network environment.
Existing wireless network technologies can be grouped into two distinct categories in terms of bandwidth capacity and coverage range; i.e. low-bandwidth connectivity over a wide geographic range and high-bandwidth connectivity over a narrow geographic range. A MH with multiple network interfaces is capable of connecting to different wireless networks simultaneously when the MH happens to be in an overlapping coverage area of the different wireless networks. Generally, when a MH initiates a session (TCP or UDP) over a low-bandwidth network with wide coverage range and then moves into an overlapping area of a high-bandwidth network with narrow coverage range, the MH continues with the low-bandwidth network despite the higher bandwidth availability. This is a because of the historical dependence of network access, a phenomenon known as network hysteresis. Clearly, network hysteresis has an adverse impact on system throughput, impairing system performance.
Problems related to HTTP session continuity in a heterogeneous wireless environment have not been widely studied. This is especially true for in-vehicle applications. This lack of study may be attributed to the obvious risk of driving while web surfing among other factors. However, the emergence of new telematics applications, especially, a variety of rear-seat applications, make this a technology area worth exploring.
A review of related work on session continuity with a focus on Mobile IP (MIP) and Session Initiation Protocol (SIP) is warranted. Initially, a distinction between two levels of granularity of HTTP session continuity is provided by the following terms; transaction-level granularity and packet-level granularity.
At transactional-level granularity, a HTTP session is indivisible for a given point of attachment. This means a HTTP session cannot be partially successful. This results in inefficiency of network utilization because a failure recovery requires that a file be downloaded from scratch. Frequent handoffs could result in frequent failure recovery, forming an endless loop of downloading of the same file. This represents a serious detriment for mobile wireless applications.
At packet-level granularity, a HTTP session is divisible with respect to the point of attachment, and can be pieced together via different points of attachment. This means that a file does not need to be restarted from scratch upon failure. This improves network utilization and is an important feature needed to optimize the over-the-air efficiency in highly mobile and heterogeneous wireless network environments.
MIP is a network-layer approach for dealing with mobility and session continuity in a mobile environment and is implemented via IP-in-IP encapsulation, IP tunneling, and IP decapsulation. Using a MIP scheme, a MH is assigned both a fixed IP (primary IP) or home address and also a care-of address that may be changed for different points of attachment. In particular, when the MH moves into a foreign network coverage area and connects to such foreign network, a care-of address is assigned for that foreign network This care-of address is registered with the home agent (HA) in the home network. The HA is responsible for tunneling IP packets by performing IP-in-IP encapsulation to a foreign agent (FA) in the foreign network, and the FA is responsible for decapsulating the packet and delivering it to the MH.
One advantage of a MIP scheme is that the device home IP address does not have to be changed each time it connects to a foreign network. Further, MIP is attractive because it is application transparent. However, these benefits comes at a price; i.e. MIP requires performing run-time IP encapsulation and decapsulation for each IP packet, and introduces costly triangle routing, as well as an additional delay between HA and FA.
Session Initiation Protocol (SIP) is a standard application-layer signaling protocol and is widely considered as a replacement of H.323 protocol for multimedia streaming and various UDP-based applications such as 3 Internet conferencing, telephony, instant messaging and real-time event notification. SIP is implemented on top of UDP or TCP, and does not support HTTP session continuity. Therefore, a transparent, extensible session-layer architecture, e.g. an interposition toolkit for generic session-layer network services, must be provided to address the end-to-end session migration for mobility across network disconnects and to provide for automatic failure recovery in the presence of network handoffs. This additional layer resides between the application-layer and the network-layer. A major disadvantage of this approach is that existing software applications must be modified to take advantage of the built in functions of SIP, and must be recompiled with a special library (software library module). This is impractical for several reasons, including the inability to access software source code so that modifications can be made, and the inability to convince the software owners to make the modifications themselves.
There are several disadvantages related to the use of MIP and SIP in heterogeneous network environments. In particular, as noted above, access to a full range of wireless networks must be available. This may not be feasible because service providers are reluctant to allow access to their networks, and because a vertical handoff normally takes more than 20 seconds, causing an unrecoverable TCP failure. Further, in a standard MIP system architecture, both FA and HA are required to be in the same network domain. Therefore, vertical handoff will not work if HA and FA are in different network domains, primarily because of the presence of network firewalls. Similarly, standard SIP protocol assumes that the SIP user agent and SIP proxy are in the same network domain. The presence of firewalls makes both SIP and MIP virtually useless for heterogeneous network environments consisting of different networks owned by different network operators, without specific business and technical arrangements allowing traffic to flow freely between the two networks.
Moreover, vertical handoff requires that a HA be able to multicast data packets to a group of base stations in which the FA resides. Firewalls and other service provider issues may constrain such multicasting. These disadvantages highlight the importance of carrier-independent solutions. In addition, for in-vehicle applications, other problems must be overcome, including transient network failures and disconnects which result from the existence of blind coverage spots and vehicle speed.
One problem arises from the fact that web servers are intrinsically stateless and each request is processed without any knowledge of previous requests. As a result, any network failure will disrupt an ongoing HTTP sessions and require the user to manually reestablish a connection to the same server. This is tolerable in the wired network, but it is not feasible for the wireless in-vehicle environment for several reasons. In particular, the current systems for the wireless in-vehicle environment do not provide for automatic failure recovery with minimum human intervention. Further, the current systems require a complete reload of a file following a network disconnect, and do not allow the affected HTTP session to be restarted from the point of disconnect. In addition, current systems do not possess network environment awareness and therefore can not overcome the problem of network hysteresis noted above.
Therefore, there remains a need in the art for improvements in the field of in-vehicle wireless technology.